Nonaqueous electrolyte batteries, in which a carbon material or a lithium-titanium oxide is used as a negative electrode active material and a layered oxide including nickel, cobalt, manganese, and the like is used as a positive electrode active material, lithium secondary batteries, in particular, have already been put to practical use as a power source in a wide range of fields. Modes of such nonaqueous electrolyte batteries range from small-sized batteries for various electronic devices to large-sized batteries for electric automobiles and the like. In an electrolyte solution for such lithium secondary batteries, unlike a nickel-hydrogen battery or a lead storage battery, used is a nonaqueous organic solvent, in which ethylene carbonate, methyl ethyl carbonate, and the like are mixed. The electrolyte solution using such a solvent has oxidation resistance and reduction resistance that is higher than those of an aqueous electrolyte solution, and thus the electrolysis of the solvent hardly occurs. For that reason, the nonaqueous lithium secondary battery can realize a high electromotive force of 2 V to 4.5 V.
On the other hand, many of the organic solvents are flammable materials, and thus the safety of the nonaqueous lithium secondary battery is apt to be inferior to the secondary battery using an aqueous solution, in principle. Although there are various measures being taken to improve the safety of the lithium secondary battery using the electrolyte solution of the organic solvent, such measures are not necessarily sufficient. Furthermore, for the nonaqueous lithium secondary battery, a dry environment is necessary in the production process, and thus the production cost is consequently increased. In addition, the electrolyte solution of the organic solvent has inferior conductivity, and thus the internal resistance of the nonaqueous lithium secondary battery is apt to increase. These issues have become big defects in applications for electric automobiles and hybrid electric automobiles, in which the battery safety and the battery cost are emphasized, and in an application for a large-sized storage battery for electricity storage.
In order to solve these issues, conversion of the electrolyte solution to an aqueous solution is being put into consideration. In an aqueous electrolyte solution, it is necessary to limit a potential range within which charging and discharging of the battery is performed to be within a potential range where an electrolysis reaction of water, included as a solvent, does not occur. For example, when a lithium-manganese oxide is used as a positive electrode active material and a lithium-vanadium oxide is used as a negative electrode active material, the electrolysis of the aqueous solvent can be avoided. According to this combination, however, though an electromotive force of about 1 V to 1.5 V can be obtained, it is difficult to obtain an energy density sufficient for a battery.
When a lithium-manganese oxide is used as the positive electrode active material and a lithium-titanium oxide such as LiTi2O4 or Li4Ti5O12 is used as the negative electrode active material, an electromotive force of about 2.6 V to 2.7 V can be theoretically obtained, and thus the battery can be expected to be attractive in terms of the energy density. In a nonaqueous lithium ion battery employing such a combination of the positive and negative electrode materials, excellent life performance can be obtained, and such a battery described has already been put into practical use. In an aqueous electrolyte solution, however, since a potential of lithium insertion and extraction for the lithium-titanium oxide is about 1.5 V (vs. Li/Li+) relative to a lithium reference potential, electrolysis of the aqueous electrolyte solution is apt to occur. In particular, at the negative electrode, hydrogen is vigorously generated by the electrolysis occurring on a surface of a negative electrode current collector or a metal outer can electrically connected to the negative electrode, and thus the active material may become easily flaked off from the current collector due to the influence of the hydrogen generation. As a result, such a battery does not function stably and sufficient charge and discharge cannot be performed.